The invention relates to fused heterocyclic compounds which inhibit protein kinase enzymes, compositions which contain protein kinase inhibiting compounds and methods of using inhibitors of protein kinase enzymes to treat diseases which are characterized by an overexpression or upregulation of protein kinases. Protein kinases mediate intracellular signal transduction. They do this by effecting a phosphoryl transfer from a nucleoside triphosphate to a protein acceptor that is involved in a signaling pathway. There are a number of kinases and pathways through which extracellular and other stimuli cause a variety of cellular responses to occur inside the cell. An extracellular stimulus may effect one or more cellular responses related to cell growth, migration, differentiation, secretion of hormones, activation of transcription factors, muscle contraction, glucose metabolism, control of protein synthesis and regulation of cell cycle.
Many diseases are associated with abnormal cellular responses triggered by protein kinase-mediated events. These diseases include autoimmune diseases, inflammatory diseases, neurological and neurodegenerative diseases, cancer, cardiovascular diseases, allergies and asthma, Alzheimer's disease or hormone-related diseases. Accordingly, there has been a substantial effort in medicinal chemistry to find protein kinase inhibitors that are effective as therapeutic agents.
Serine/threonine kinases are a class of protein kinases that are among the most promising drug targets for future small molecule inhibitors. Inhibition of serine/threonine kinases is likely to have relevance to the treatment of cancer, diabetes and a variety of inflammatory disorders. The successful development of GLEEVEC® as a Bcr/Abl protein kinase inhibitor has provided further evidence that protein kinases including protein kinase CK2 are valid drug targets for potential cancer therapies.
Protein kinase CK2 (formerly known as casein kinase II) is a highly conserved serine/threonine kinase. Protein kinase CK2 is ubiquitously distributed and constitutively active in eukaryotes. In mammals, the enzyme exists in two isozymic forms due to variations in the catalytic subunits of the enzyme. The CK2 holoenzyme is a heterotetrameric complex composed of two catalytic α (CK2A1) subunits or α′ (CK2A2) subunits and two regulatory β-subunits. The formation of CK2 complexes containing the catalytic subunits requires dimerization of the regulatory β-subunits. CK2 interacts with a variety of cellular proteins and has been implicated in cell replication such as cell proliferation and differentiation, cellular survival, and tumorigenesis. With respect to tumorigenesis, protein kinase CK2 has been implicated in kidney tumors (Stalter et al., “Asymmetric expression of protein kinase CK2 subunits in human kidney tumors”, Biochem. Biophys. Res. Commun., 202:141-147 (1994)), mammary gland tumors (Landesman-Bollag et al., “Protein kinase CK2 in mammary gland tumorigenesis”, Oncology, 20:3247-3257 (2001)), lung carcinoma (Daya-Makin et al., “Activation of a tumor-associated protein kinase (p40TAK) and casein kinase II in human squamous cell carcinomas and adenocarcinomas of the lung”, Cancer Res., 54:2262-2268 (1994)), head and neck carcinoma (Faust et al., “Antisense oligonucleotides against protein kinase CK2-α inhibit growth of squamous cell carcinoma of the head and neck in vitro”, Head Neck, 22:341-346 (2000)), and prostate cancer (Wang et al., “Role of protein kinase CK2 in the regulation of tumor necrosis factor-related apoptosis inducing ligand-induced apoptosis in prostate cancer cells”, Cancer Res., 66:2242-2249 (2006)).
Inhibitors of protein kinases are widely sought and small molecule compounds capable of modulating protein kinases have been reported. For example, pyrazolotriazines as CK2 kinase inhibitors were reported in Nie et al. (Bioorganic & Medicinal Chemistry Letters, 17:4191-4195 (2007); 18:619-623 (2008)) and imidazopyridazines as IRAK kinase modulators were reported in PCT Publication WO 2008/030579. In addition, certain imidazopyridazine compounds were disclosed in WO 2007/038314, published Apr. 5, 2007, WO 2008/0045536, published Feb. 21, 2008, both assigned to the present assignee. The present invention relates to a new class of imidazopyridazine-carbonitriles found to be effective inhibitors of protein kinases, particularly the CK2 kinase. These novel compounds are provided to be useful as pharmaceuticals with desirable stability, bioavailability, therapeutic index and toxicity values that are important to their drugability.